The invention relates to an optical recording element having a transparent supporting plate at least one side of the surface of which is manufactured from a synthetic resin or has a coating layer of a synthetic resin and a recording layer of a chalcogenide provided on the said side.
Such an optical recording element is known from Netherlands Patent Application No. 80 05 693 in the name of N. V. Philips' Gloeilampenfabrieken.
This known recording element comprises a transparent supporting plate of, for example, glass which has a layer of a cross-linked synthetic resin on one side. An optically readable servo track is provided in the synthetic resin layer in the form of a groove which may be provided with a finely detailed structure of servo areas the dimensions of which are in the order of magnitude of from one to a few microns. A recording layer of a chalcogenide is provided on the layer of synthetic resin. The chalcogenide layer in particular is a tellurium alloy, for example, an alloy of Te, Se, Sb and S. Data are recorded optically in the recording layer. For this purpose the optical recording element is rotated at, for example, a frequency of 4 Hz and the recording layer is exposed, via the transparent supporting plate, to energy-intensive laser light which is pulsated in accordance with the data to be recorded. The laser light follows the servo track. The recording material melts in the exposed places and a cavity or hole having a thickened edge portion is formed under the influence of the surface tension. Said information bits are arranged so as to form an information track which usually is spiral-like or consists of concentric circles. At the area of the holes the transmission of the recording layer for laser light is increased and the reflection is reduced. On the basis of this the information bits can be read with comparatively weak laser light. The known recording element has a great sensitivity to laser light. For example, the threshold energy of the recording layer Te.sub.60 Se.sub.25 Sb.sub.10 S.sub.5 is approximately 0.5 nJ measured at a wavelength of 633 nm, a frequency of 4 Hz and a numerical aperture of the objective which focuses the laser light of 0.45. The threshold energy is the minimum energy required to provide a cavity or hole per laser light pulse. The pulse period is, for example, 100 ns.
Applicants have found that the quality of the above-mentioned sensitive recording element decreases in the course of time. For example, the signal-to-noise ratio decreases. Errors have also been established in tracking the servo track, in writing information and in reading same.
It has been found that the deterioration of the quality and the errors found are the result of the optical reading of the recorded data. During reading a continuous laser light beam is used, i.e. a non-pulsated light beam which is focused on the information track via the transparent supporting plate. The power of the laser upon reading is considerably smaller than that of the laser upon writing and is, for example, a factor 10 weaker. For example, the reading power is at most 1.3-1.4 mW and is preferably 0.4-1.3 mW. It has been found however, that upon reading, variations occur in the recording layer or in the synthetic resin surface adjoining said layer in spite of the low reading power.
It has become evident from investigations underlying the invention that the reading power used is very critical. For example, there exists a threshold-reading power above which variations are induced in the recording layer and/or the synthetic resin surface. Even different threshold powers can be distinguished independently of the manner of reading and of the nature of the variations. The following example serves for illustration.
The above-mentioned recording layer of Te.sub.60 Se.sub.25 Sb.sub.10 Sb.sub.5 having a thickness of 23 nm has a crystalline structure which has been obtained by subjecting the layer to a thermal treatment during or after the manufacture. Upon reading information bits provided in said layer, no changes occur in the layer up to a reading power of approximately 0.8 mW.
With a reading power of approximately 0.8 to 1.0 mW changes are induced in the recording layer if the information track or a part of the information track, for example one track thereof, is read continuously for a longer period of, for example, 1 minute to 30 minutes, or if the track or a part thereof is read very frequently, for example, is read 250 to 7,000 times during the above-mentioned periods of 1 to 30 minutes. The changes are substantially a local transition from the crystalline recording material to a lesser ordered phase, in particular an amorphous phase. The amorphous spots in the recording material have optical properties different from the crystalline portions and, for example, are more transparent. As a result of this the quality of the recorded data and the quality of the reading (play-back) is detrimentally influenced. The signal-to-noise ratio decreases drastically.
With a reading power of approximately 1.0-1.2 mW damage occurs to the recording layer during continuous or frequent reading, the layer locally melting and becoming more transparent to laser light. Damage also occurs to the synthetic resin material on which the recording layer has been provided. The very finely structured servo track which is provided in the synthetic resin surface is damaged in particular. Tracking and recording of data is no longer possible.
With a reading power of approximately 1.2 to 1.5 mW, the conversion from crystalline to amorphous and the damage to the recording layer and the synthetic resin material (servo track) already occur after only one or a few readings.
The great sensitivity to damage and conversions from the crystalline to the amorphous state on frequent or continuous reading by means of a weak laser light beam are caused in Applicants' opinion particularly by the synthetic resin surface of the supporting plate or the synthetic resin layer on the supporting plate. Experiments have demonstrated that if the recording layer of, for example, Te.sub.60 Se.sub.25 Sb.sub.10 S.sub.5 is provided directly on the glass surface of the supporting plate, the threshold power on frequent reading is 2.5 mW for the conversion from crystalline to amorphous and is 3 mW for small damage. A 2-3 fold increase of the threshold. In this case no problems occur on frequent reading with, for example, a laser power of at most 1.5 mW. The disadvantage is that no auxiliary track (servo track) can be provided in such an element. A further disadvantage is that the threshold sensitivity for the recording of information is more than doubled so that the useful life of the laser, in particular of the AlGaAs laser which in itself is interesting, is considerably reduced.